The miniaturization of semiconductor devices and the like is advantageous in bringing about an improvement in performance and function (higher-speed operation, lower power consumption, etc.) and a reduction in cost and thus has been accelerated more and more. The lithography technique has been supporting this miniaturization and transfer masks are a key technique along with exposure apparatuses and resist materials.
In recent years, the development of the DRAM half-pitch (hp) 45 nm to 32 nm generations according to the semiconductor device design rule has been progressing. This corresponds to about ¼ to ⅙ of a wavelength 193 nm of ArF excimer laser exposure light (hereinafter referred to as “ArF exposure light”). Particularly, in the DRAM hp45 nm and subsequent generations, only the application of the resolution enhancement technology (RET) such as the conventional phase shift method, oblique incidence illumination method, and pupil filter method and the optical proximity correction (OPC) technique has been becoming insufficient and the hyper-NA (numerical aperture) technique (immersion lithography) has been becoming necessary.
In the meantime, circuit patterns necessary in the manufacture of semiconductor devices are formed in sequence by exposing a photomask (reticle) pattern a plurality of times onto a semiconductor wafer. For example, a reduced projection exposure apparatus (exposure apparatus) with a predetermined reticle set therein repeatedly projects and exposes a pattern of the reticle while sequentially shifting a projection area on a semiconductor wafer (step-and-repeat system), or repeatedly projects and exposes a pattern of the reticle while synchronously scanning the reticle and a semiconductor wafer with respect to a projection optical system (step-and-scan system). These systems have been predominant. As a consequence, a predetermined number of integrated circuit chip areas are formed in the wafer.
A photomask (reticle) has an area formed with a transfer pattern and a peripheral area around the transfer pattern area, i.e. an edge area along four sides in the photomask (reticle). When exposing the transfer pattern of the photomask (reticle) while sequentially shifting a projection area on a semiconductor wafer, the transfer pattern is exposed and transferred to the projection areas so that the photomask peripheral areas overlap each other for the purpose of increasing the number of integrated circuit chips to be formed. Normally, a mask stage of an exposure apparatus is provided with a shielding plate for blocking irradiation of exposure light onto the photomask peripheral area. However, in the case of blocking the irradiation of the exposure light by the shielding plate, there are problems of positional accuracy limitation and of light diffraction phenomenon so that it is not possible to avoid leakage of the exposure light to the photomask peripheral area (hereinafter, this exposure light will be referred to as “leakage light”). If this leakage light to the photomask peripheral area is transmitted through the photomask, there is a possibility of sensitizing a resist on the wafer.
In order to prevent the sensitization of the resist on the wafer due to such overlapping exposure, a light-shielding band (a band of shielding material or light-shielder ring) is formed in the photomask peripheral area by mask processing. Normally, it is reported that, in an area, where the light-shielding band is formed, of the photomask peripheral area, an OD value (optical density) of 3 or more is desirable and that of at least about 2.8 is necessary for preventing the sensitization of the resist on the wafer due to the overlapping exposure.
In the case of a binary mask, since the light-shielding performance of a light-shielding film is high, the light-shielding film serves to form a light-shielding film pattern in a transfer pattern area and further to form a light-shielding band in a peripheral area around the transfer pattern area.
The light-shielding film is also required to have a certain low front-surface reflectance for exposure light. In view of this, the light-shielding film generally has a laminated structure of at least two layers, i.e. a layer for ensuring the light-shielding performance and a layer (front-surface antireflection layer) for reducing the front-surface reflectance. The front-surface antireflection layer has a difficulty in enhancing the light-shielding performance in terms of its properties and thus cannot contribute to a reduction in the thickness of the light-shielding film. The reduction in the thickness of the light-shielding film has these restrictions.
When the thickness of the light-shielding film is reduced, the OD (optical density) value is also reduced. In the case of a chromium-based light-shielding film, the total thickness of about 60 nm is minimally required for achieving OD=3 which is generally required, and therefore, a large reduction in the thickness of the film is difficult to achieve (see, e.g. Patent Document 1: JP-A-2007-241136, paragraph [0005]).
Further, also in the case of, for example, a so-called binary photomask comprising a light-shielding film having a laminated structure of MoSi-based materials, such as a light-shielding film having a laminated structure of a MoSiN light-shielding layer and a MoSiON antireflection layer from the substrate side, the total thickness of normally about 60 nm is minimally required for achieving OD=2.8 which is required, and therefore, a large reduction in the thickness of the film is difficult to achieve (Patent Document 2: JP-A-2006-78825).
On the other hand, Patent Document 3 (WO2005/124464) discloses a mask blank comprising a light-semitransmissive film. This light-semitransmissive film has a property of transmitting exposure light at a predetermined transmittance and this property is substantially the same as that of a conventional halftone phase shift film. However, this light-semitransmissive film also has a property such that the phase difference between exposure light transmitted through a light-semitransmissive portion formed with the light-semitransmissive film and exposure light transmitted through a light-transmissive portion formed with no light-semitransmissive film is small. This property is totally different from that of the conventional halftone phase shift film. The mask blank comprising this light-semitransmissive film is used for manufacturing an enhancer mask.